Planar waveguides with enhanced support and/or cooling features for high-power laser systems

ABSTRACT

This disclosure provides planar waveguides with enhanced support and/or cooling. One or more endcaps could be disposed between coating/cladding layers at one or more ends of a core region, where the core region is doped with at least one active ion species and each endcap is not doped with any active ion species that creates substantial absorption at pump and signal wavelengths. A core region could include at least one crystal or crystalline material, and at least one cladding layer could include at least one glass. Different types of coolers could be disposed on or adjacent to different coating/cladding layers. Side claddings could be disposed on opposite sides of a planar waveguide, where the opposite sides represent longer sides of the waveguide. Endcaps and one or more coolers could be sealed to a housing, and coolant can flow through a substantially linear passageway along a length of the waveguide. One side of a planar waveguide could be uncooled.

TECHNICAL FIELD

This disclosure is generally directed to high-power laser systems. Morespecifically, this disclosure is directed to planar waveguides withenhanced support and/or cooling features for high-power laser systems.

BACKGROUND

High-power laser systems are being developed for a number of militaryand commercial applications. However, at high average output powers,problems associated with a lasing medium in a laser system areexacerbated. These problems include stress in the mounting of the lasingmedium, inefficient and non-uniform cooling of the lasing medium,inefficient coupling of pumplight into the lasing medium, andinefficient coupling of laser beams into and out of the lasing medium.

Low-efficiency cooling may cause an unacceptable rise in the averagetemperature of the lasing medium relative to an inlet temperature of acoolant. Mounting stress and non-uniform heating can create phasedistortions in an amplified output beam and can lead to damage or evenfracturing of the lasing medium, which can also damage nearby elementsof the laser system. Inefficient coupling of pumplight into the lasingmedium can lead to overall laser inefficiencies, and stray pumplight cancause damage to the lasing medium or other elements of the laser system.Stray input or output laser beams can damage elements of the lasersystem, and the loss of power detracts from a total output power and anoverall laser efficiency of the laser system.

SUMMARY

This disclosure provides planar waveguides with enhanced support and/orcooling features for high-power laser systems.

In a first embodiment, an apparatus includes a planar waveguideconfigured to receive and amplify optical signals. The planar waveguideincludes a core region, first and second coating or cladding layersdisposed on opposite sides of the core region, and one or more endcapsdisposed between the first and second coating or cladding layers at oneor more ends of the core region. The core region includes at least onematerial and is doped with at least one active ion species, and eachendcap includes the at least one material and is not doped with anyactive ion species that creates substantial absorption at pump andsignal wavelengths.

In a second embodiment, an apparatus includes a planar waveguideconfigured to receive and amplify optical signals. The planar waveguideincludes a core region and at least one cladding layer disposed on thecore region. The core region includes at least one crystal orcrystalline material, and the at least one cladding layer includes atleast one glass.

In a third embodiment, an apparatus includes a planar waveguideconfigured to receive and amplify optical signals. The planar waveguideincludes a core region and first and second coating or cladding layersdisposed on opposite sides of the core region. The apparatus alsoincludes a first cooler disposed on or adjacent to the first coating orcladding layer and configured to cool the planar waveguide and a secondcooler disposed on or adjacent to the second coating or cladding layerand configured to cool the planar waveguide. The first and secondcoolers are different types of coolers.

In a fourth embodiment, an apparatus includes a planar waveguideconfigured to receive and amplify optical signals. The planar waveguideincludes a core region and at least one coating or cladding layerdisposed on the core region. The apparatus also includes first andsecond side claddings disposed on opposite sides of the planarwaveguide, where the opposite sides represent the longer sides of theplanar waveguide.

In a fifth embodiment, an apparatus includes a planar waveguideconfigured to receive and amplify optical signals. The planar waveguideincludes a core region and at least one coating or cladding layerdisposed on the core region. The apparatus also includes first andsecond endcaps sealed to the planar waveguide and to a housing and oneor more coolers sealed to the housing and configured to cool the planarwaveguide. At least one of the one or more coolers is configured toreceive a coolant and to direct the coolant through a passageway definedwithin an interior space of the housing, where the passageway includes asubstantially linear passageway along a length of the planar waveguide.

In a sixth embodiment, an apparatus includes a planar waveguideconfigured to receive and amplify optical signals. The planar waveguideincludes a core region and first and second coating or cladding layersdisposed on opposite sides of the core region. The apparatus alsoincludes a cooler disposed on or adjacent to the first coating orcladding layer and configured to cool the planar waveguide. The secondcoating or cladding layer is uncooled.

Other technical features may be readily apparent to one skilled in theart from the following figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is madeto the following description, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates an example high-power laser system according to thisdisclosure;

FIG. 2 illustrates an example planar waveguide (PWG) for use in ahigh-power laser system according to this disclosure;

FIG. 3 illustrates an example PWG pumphead assembly of a high-powerlaser system according to this disclosure;

FIGS. 4A and 4B illustrate an example PWG cartridge for a PWG pumpheadassembly of a high-power laser system according to this disclosure; and

FIGS. 5 through 12 illustrate example arrangements for supporting and/orcooling a planar waveguide of a high-power laser system according tothis disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 12, described below, and the various embodiments used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the present invention may beimplemented in any type of suitably arranged device or system.

FIG. 1 illustrates an example high-power laser system 100 according tothis disclosure. As shown in FIG. 1, the laser system 100 includes amaster oscillator 102 and a planar waveguide (PWG) amplifier 104. Thelaser system 100 therefore has a master oscillator/power amplifier(MOPA) configuration.

The master oscillator 102 generally operates to generate low-poweroptical signals. The low-power optical signals could denote any suitableoptical signals having relatively low power. For example, the low-poweroptical signals could include optical signals having a continuous wave(CW) output, a continuous pulse train (CPT), a pulse burst, or any ofvarious other waveforms. The master oscillator 102 includes any suitablestructure(s) for generating one or more low-power optical signals. Insome embodiments, the master oscillator 102 includes a fiber laser.

The PWG amplifier 104 receives the low-power optical signals from themaster oscillator 102 and pump power. The PWG amplifier 104 generallyoperates to amplify the low-power optical signals and generatehigh-power optical signals. For example, the PWG amplifier 104 couldamplify a low-power CW or other optical signal into a high-power CW orother optical signal having ten kilowatts of power or more. The receivedpump power provides the necessary population inversion in the PWGamplifier's gain medium for this amplification.

The gain medium of the PWG amplifier 104 is formed using a planarwaveguide. A planar waveguide generally denotes a structure thatincludes a core region and one or more coating or cladding layers. Thecore region is doped with an active ion species responsive to opticalsignals of at least one specific wavelength, and the one or more coatingor cladding layers are optically transparent and contact the coreregion. The indexes of refraction and dielectric constants of the coreregion and the coating or cladding layer(s) differ and create boundariesthat reflect the optical signals. The planar waveguide thereforeoperates to guide optical signals in its narrower dimension (referred toas the “fast axis” direction) but not in its broader dimension (referredto as the “slow axis” direction). The planar waveguide could be formedfrom any suitable materials and in any suitable manner.

Relay optics 106 direct the optical signals from the master oscillator102 into a beam controller 108, and relay optics 110 direct the opticalsignals from the beam controller 108 into the PWG amplifier 104. Therelay optics 106 and 110 can also alter the cross-sectional dimensionsof the optical signals as needed for injection into the beam controller108 and the PWG amplifier 104, respectively. Each of the relay optics106 and 110 includes any suitable optical device(s) for directing orformatting optical signals.

The beam controller 108 generally operates to modify the optical signalsfrom the master oscillator 102 before the optical signals reach the PWGamplifier 104. For example, the beam controller 108 could pre-distortthe optical signals from the master oscillator 102 in order tosubstantially or completely compensate for distortions created withinthe PWG amplifier 104. The beam controller 108 includes any suitablestructure(s) for pre-distorting or otherwise modifying optical signalsin a controllable manner.

The high-power output beam generated by the PWG amplifier 104 isdirected towards a beam splitter 112. The beam splitter 112 provides asubstantial portion of the high-power output beam to relay optics 114,which provide that portion of the high-power output beam out of thelaser system 100 as a high-power output beam 116. The beam splitter 112also provides a small amount of the high-power output beam as samples toa feedback loop. The feedback loop is used to control and modifyoperation of the master oscillator 102, the PWG amplifier 104, and thebeam controller 108. The beam splitter 112 includes any suitablestructure(s) for splitting optical signals. The relay optics 114 includeany suitable optical device(s) for directing or formatting opticalsignals.

The feedback loop here includes a diagnostics unit 118, a lasercontroller 120, and diode drivers 122-124. The diagnostics unit 118generally operates to analyze the samples of the high-power output beamfrom the PWG amplifier 104 and provide diagnostic data to the lasercontroller 120. The diagnostics unit 118 includes any suitable structurefor identifying one or more characteristics of at least one sample of ahigh-power output beam. In some embodiments, diagnostic data from thediagnostics unit 118 could be output by the laser controller 120, suchas to one or more external destinations. This could allow the externaldestinations to monitor the health, status, or safety of the lasersystem 100.

The laser controller 120 generally operates to control the operation ofthe beam controller 108 and the diode drivers 122-124. The lasercontroller 120 includes any suitable structure for controlling operationof a laser system. For example, the laser controller 120 could includeone or more processing devices, such as one or more microprocessors,microcontrollers, digital signal processors, field programmable gatearrays, application specific integrated circuits, or discrete logicdevices. The laser controller 120 could also include one or morememories configured to store instructions or data used, generated, orcollected by the processing device(s). The laser controller 120 couldfurther include one or more interfaces configured to facilitatecommunications with other components or systems.

The diode driver 122 generates electrical drive signals that cause oneor more laser diodes (or other light sources) of the master oscillator102 to generate optical pump power for the master oscillator 102, whichcauses the master oscillator 102 to generate desired low-power opticalsignals. The diode driver 124 generates electrical drive signals thatcause laser diodes (or other light sources) of the PWG amplifier 104 togenerate optical pump power for the PWG amplifier 104, which uses thepump power to provide optical amplification. The diode driver 124 couldbe capable of operation across a range of input voltages and loadconditions while protecting expensive laser diode strings fromelectrical shorts and transients. Each diode driver 122-124 includes anysuitable structure(s) for driving any suitable arrangement of laserdiodes or other light sources.

The laser system 100 can incorporate a number of novel features, whichare described below. These novel features can be used individually or inany suitable combination. Various other features can also beincorporated into the laser system 100. For example, the laser system100 could use the techniques disclosed in U.S. patent application Ser.No. 14/661,828 and U.S. Patent Publication No. 2014/0268309 (which arehereby incorporated by reference) to cool various components of thelaser system 100, including the use of a thermal-optic interface (TOI)material. The laser system 100 could use the techniques disclosed inU.S. Patent Publication No. 2014/0268309 to suppress amplifiedspontaneous emission (ASE) within a PWG device, including the use ofbeveled edges on the PWG device. The laser system 100 could use thetechniques disclosed in U.S. patent application Ser. No. 14/682,539(which is hereby incorporated by reference), including the use ofsymmetric and asymmetric core regions in PWG devices. The laser system100 could use the techniques disclosed in U.S. patent application Ser.No. 14/749,398 (which is hereby incorporated by reference), includingthe use of asymmetric two-layer PWG devices. The PWG amplifier 104 ofthe laser system 100 could be formed using the techniques disclosed inU.S. patent application Ser. No. 14/845,916 (which is herebyincorporated by reference), including the use of radio frequency (RF)magnetron sputtering and other techniques to form PWG-based laserdevices.

Overall, the laser system 100 combines the benefits of fiber lasers(such as high gain with good beam quality) and bulk solid-state lasers(such as scalability) in an overall laser architecture that supports thegeneration of high-power laser outputs. The PWG amplifier 104 functionsas a lasing element that facilitates high gain over a long amplifierpath while simultaneously maintaining near diffraction-limited beamquality in one dimension (the fast axis of the PWG amplifier 104).Substantially uniform pumping of the lasing medium and substantiallyuniform cooling of the core region combine to produce a reasonablyhigh-quality beam in the transverse dimension (the slow axis of the PWGamplifier 104).

Although FIG. 1 illustrates one example of a high-power laser system100, various changes may be made to FIG. 1. For example, any singlefeature or combination of features described above and/or below could beused in the laser system 100. Also, the functional divisions shown inFIG. 1 are for illustration only. Components could be combined, furthersubdivided, omitted, or rearranged or additional components could beadded according to particular needs.

FIG. 2 illustrates an example planar waveguide 200 for use in ahigh-power laser system according to this disclosure. The planarwaveguide 200 could, for example, be used in the PWG amplifier 104 ofthe high-power laser system 100.

As shown in FIG. 2, the planar waveguide 200 includes a core region 202and one or more coating or cladding layers 204 a-204 b. The core region202 is doped with an active ion species responsive to optical signals ofat least one specific wavelength, and the one or more coating orcladding layers 204 a-204 b are optically transparent and contact thecore region 202. The core region 202 could be formed from any suitablematerial(s), such as doped yttrium aluminum garnet (YAG). Each claddinglayer 204 a-204 b could also be formed from any suitable material(s),such as glass or YAG. A number of additional features, such as ananti-reflective coating on input or output face(s) of the coating orcladding layers 204 a-204 b, could be used.

A signal beam 206 from the master oscillator 102 is coupled into thecore region 202 of the planar waveguide 200, and pumplight 208 iscoupled into one or more of the coating or cladding layers 204 a-204 bto provide pump power for optical amplification. The indexes ofrefraction and dielectric constants of the core region 202 and thecoating or cladding layer(s) 204 a-204 b differ and create boundariesthat reflect the optical signals of the signal beam 206. The planarwaveguide 200 therefore operates to guide the optical signals of thesignal beam 208 in the “fast axis” or “guided axis” direction but not inthe “slow axis” or “unguided axis” direction. Amplification is providedalong the longer axis of the planar waveguide 200 (referred to as the“lasing axis”).

The signal beam 206 is confined within the very thin guiding core region202 that is doped with a lasing species, and the signal beam 206 isamplified via stimulated emission. The pumplight 208 is injected intothe coating or cladding layer(s) 204 a-204 b on one or both sides of thecore region 202, and the pumplight 208 is confined and efficientlycoupled into the core region 202 along the entire length of the planarwaveguide 200. The thickness of the core region 202 and the refractiveindex difference between the core region 202 and the coating or claddinglayer(s) 204 a-204 b define a numerical aperture that supports only thelowest-order propagating mode. This helps to ensure high beam quality inthe thin dimension (fast axis) of the planar waveguide 200.

In a MOPA configuration, the signal beam 206 from the master oscillator102 is injected into one end of the active core region 202, where it isamplified in one or multiple passes through the planar waveguide 200.Since the signal beam 206 is unguided in the wide dimension (slow axis),the signal beam 206 is susceptible to thermal lensing effects due tonon-uniform temperature distributions across the planar waveguide 200.It is also susceptible to depolarization due to thermally-induced stressbirefringence. Stress may further produce wavefront distortions in thesignal beam 206, depending on the strength of the stress-optic tensor inthe lasing medium that additionally distorts the signal beam 206.

This disclosure provides various embodiments of planar waveguides thatmay improve the performance, reliability, and/or cost of a PWG laser.This can be accomplished by providing improved mechanisms for supportingand/or cooling a planar waveguide. Example embodiments providing thesesupporting and/or cooling mechanisms are described below.

Although FIG. 2 illustrates one example of a planar waveguide 200 foruse in a high-power laser system, various changes may be made to FIG. 2.For instance, the size, shape, and dimensions of the various elementsshown in FIG. 2 are for illustration only and can vary as needed ordesired. As a particular example, the coating or cladding layers 204a-204 b could have substantially different thicknesses, creating anasymmetric planar waveguide. As another particular example, the coreregion 202 could have a non-uniform thickness along the lasing axis.

FIG. 3 illustrates an example PWG pumphead assembly 300 of a high-powerlaser system according to this disclosure. The pumphead assembly 300could, for example, include the planar waveguide 200 of FIG. 2 and beused in the PWG amplifier 104 of FIG. 1.

As shown in FIG. 3, the PWG pumphead assembly 300 includes a PWGpumphead housing 302 and a PWG cartridge 304. The PWG cartridge 304includes the planar waveguide 200, which receives the signal beam 206from the master oscillator 102 and the pumplight 208. The pumplight 208can be provided by any suitable source(s) 306, such as one or morearrays of laser pump diodes.

The PWG cartridge 304 includes two support blocks 308, and the planarwaveguide 200 is located between the blocks 308. The blocks 308 coulddenote coolers or other structures used to hold or encase the planarwaveguide 200. The blocks 308 could be formed from any suitablematerial(s) and in any suitable manner. In particular embodiments, eachblock 308 could denote a microchannel cooler, multi-jet liquidimpingement cooler, cooling plate, ultra-thin liquid thermal-opticalinterface cooler (such as in U.S. patent application Ser. No.14/661,828), or other structure for cooling the planar waveguide 200.

A scraper mirror 310 reflects light that is not coupled into the planarwaveguide 200. The scraper mirror 310 could form a part of or beconnected to a light pipe, which can be used to couple the pumplight 208into the cladding layer(s) of the planar waveguide 200. The light pipeor an endcap of the planar waveguide 200 can also help to homogenize thepumplight 208 prior to injection into the cladding layer(s) of theplanar waveguide 200. The scraper mirror 310 includes any suitablestructure for reflecting light. The light pipe includes any suitableoptical device for confining, homogenizing, and/or transporting opticalsignals.

In some embodiments, at least the PWG cartridge 304 and optionally othercomponents (such as the pumplight source or sources 306) are implementedas one or more replaceable cartridges for the laser system. Among otherthings, this cartridge-based approach allows different cartridge designsto be installed with common (possibly standardized) mechanical, thermal,electrical, and optical interfaces without making any changes to thepumphead and optical bench. As a particular example, different PWGcartridges 304 could include different planar waveguides and/or coolingmechanisms for the planar waveguides. Also, different design teams cancreate and test different designs for the various components of thelaser system 100. There may be little or no need for one design team towait for the development of one component of the laser system 100 to becompleted before designing and testing another component of the lasersystem 100.

In some embodiments, the PWG cartridge 304 receives a cooling fluidthrough one or more channels, such as channels 312-314. For example,each block 308 could represent a cooler, and one or more of thesecoolers could operate using a flow of cooling fluid. The cooling fluidcould be received through one or more of the cooling channels 312-314and returned through one or more others of the cooling channels 312-314.In this example, the cooling channels 312-314 on opposite sides of theplanar waveguide 200 are identical, although different arrangements ofcooling channels could be used, or cooling channels could be omitted onone or both sides of the planar waveguide 200.

The PWG cartridge 304 is held in place using one or more clamps 316. Amain body of the housing 302 could be formed from an integral piece ofmaterial, and a cover 318 can provide access to the interior of thehousing 302. The cover 318 allows a portion of each clamp 316 to extendout of the housing 302 so that the clamp 316 can be adjusted. The clamps316 push inward (down in FIG. 3) in order to secure the PWG cartridge304 in place. Each clamp 316 includes any suitable structure forapplying force against a PWG cartridge. The cover 318 includes anysuitable structure allowing passage of a portion of at least one pusherassembly.

Although FIG. 3 illustrates one example of a PWG pumphead assembly 300of a high-power laser system, various changes may be made to FIG. 3. Forinstance, the size, shape, and dimensions of the various elements shownin FIG. 3 are for illustration only and can vary as needed or desired.

FIGS. 4A and 4B illustrate an example PWG cartridge 304 for a PWGpumphead assembly of a high-power laser system according to thisdisclosure. The PWG cartridge 304 could, for example, include the planarwaveguide 200 of FIG. 2 and be used in the PWG pumphead assembly 300 ofFIG. 3.

As shown in FIGS. 4A and 4B, the PWG cartridge 304 includes the planarwaveguide 200. The planar waveguide 200 has at least one TOI material402 placed in thermal contact with the broad clad faces of the planarwaveguide 200. The exposed sides of the TOI material 402 can be bondedor otherwise attached to the blocks 308, which could represent one ormore coolers. The attachment of the planar waveguide 200 to the blocks308 could occur in any suitable manner, such as by using a thermallyconducting epoxy. The blocks 308 include channels 404 that can be usedto transport coolant to cool the PWG cartridge 304. For example, thechannels 404 can align with the channels 312-314 of the housing 302.

The TOI material 402 includes any suitable material facilitating thetransport of heat away from a planar waveguide. Each block 308 includesany suitable structure for removing heat from a planar waveguide. Insome embodiments, for example, each block 308 could denote a coppermicrochannel heatsink module.

Although FIGS. 4A and 4B illustrate one example of a PWG cartridge 304for a PWG pumphead assembly of a high-power laser system, variouschanges may be made to FIGS. 4A and 4B. For instance, the size, shape,and dimensions of the various elements shown in FIGS. 4A and 4B are forillustration only and can vary as needed or desired.

FIGS. 5 through 12 illustrate example arrangements for supporting and/orcooling a planar waveguide of a high-power laser system according tothis disclosure. The embodiments of the planar waveguides shown in FIGS.5 through 12 incorporate various features to reduce the mountingstresses on the planar waveguides and/or features to more uniformly coolthe planar waveguides. Each of these implementations or other similarimplementations could be used in the laser systems described above.

As shown in FIG. 5, a planar waveguide 500 includes a core region 502and one or more coating or cladding layers 504 a-504 b. As noted above,the core region 502 receives and amplifies a signal beam (such as thesignal beam 206), and one or more of the coating or cladding layers 504a-504 b receive pumplight (such as the pumplight 208). Note that in thisexample, the coating or cladding layers 504 a-504 b are asymmetric,which means the coating or cladding layers 504 a-504 b havesubstantially different thicknesses. In some embodiments, the thickercoating or cladding layer 504 a may provide a stable structure for thecore region 502 and can be substantially matched in its coefficient ofthermal expansion to the core region 502 so that minimal stress isplaced on the core region 502. However, the use of an asymmetric designis not required. In other embodiments, the coating or cladding layers504 a-504 b have substantially the same thickness, leading to asymmetric configuration, which could allow the temperature profile andstress to be balanced between the layers 504 a-504 b.

Passive guiding endcaps 506 a-506 b are provided at the ends of the coreregion 502 in the planar waveguide 500. In some embodiments, the endcaps506 a-506 b can be added to the core region 502 prior to formation orbonding of the coating or cladding layer(s) 504 a-504 b. The endcaps 506a-506 b could be formed from any suitable material(s) and in anysuitable manner. For example, in some embodiments, the endcaps 506 a-506b could be formed from the same material(s) as the core region 502 (suchas YAG), and the endcaps 506 a-506 b could be doped with at least onedopant to change their index of refraction. However, unlike the coreregion 502, any doping of the endcaps 506 a-506 b produces negligibleabsorption at the pump and signal wavelengths, so little or no lasingoccurs within the endcaps 506 a-506 b. In FIG. 5, the endcaps 506 a-506b are used at both ends of the planar waveguide 500 (namely an input endand an output end of the planar waveguide 500), although only one endcap506 a or 506 b could be used at one end of the planar waveguide 500.

The one or more endcaps 506 a-506 b help to provide support for theplanar waveguide 500 and reduce mounting stresses placed on the planarwaveguide 500. For example, the endcaps 506 a-506 b do not providelasing and are used for guiding optical signals, which means the endcaps506 a-506 b are not heated as much as the core region 502. Externalsupport structures could therefore be placed in contact with the endcaps506 a-506 b, such as to secure the planar waveguide 500 within a PWGcartridge 304. The reduced heating of the endcaps 506 a-506 b comparedto the core region 502 can provide a more stable and less stressfulmounting of the planar waveguide 500.

As shown in FIG. 6, a planar waveguide 600 includes a core region 602and one or more cladding layers 604 a-604 b. As noted above, the coreregion 602 receives and amplifies a signal beam, and one or more of thecladding layers 604 a-604 b receive pumplight. Note that the claddinglayers 604 a-604 b are asymmetric in this example, although a symmetricdesign could also be used.

The core region 602 in this example could be formed from a singlecrystal, a crystalline ceramic, or other crystalline material. Also, atleast one of the cladding layers 604 a-604 b could be formed from aglass. The glass may be directly fused to the core region 602, such asby applying heat after the core and cladding surfaces are preciselycontacted. Pressure may also be added during the fusing process toensure a stable, robust bond. Various other approaches could be used tofuse glass and crystalline components together.

The coefficients of thermal expansion of the glass and crystallinecomponents can be closely matched to reduce stresses on the planarwaveguide 600 during operation. The index of refraction of the glasscladding layer(s) 604 a-604 b can be different from but close to theindex of refraction of the core region 602, and the cladding layer(s)604 a-604 b can have low absorption at the wavelengths of interest.

As shown in FIG. 7, a PWG assembly 700 includes a planar waveguide 702,which in this example is implemented using the core region 602 and thecladding layers 604 a-604 b of FIG. 6 (although other planar waveguidescould be used). The planar waveguide 702 here is asymmetric, and as aresult the thinner cladding layer 604 b may increase in temperature lessthan the thicker cladding layer 604 a during operation of the planarwaveguide 702.

The PWG assembly 700 uses two different cooling interfaces for differentsides of the planar waveguide 702. Each cooling interface could beoptimized to perform desired functions or achieve specific goals, andthose functions or goals need not be identical. For example, the coolinginterface on one side of the planar waveguide 702 could be optimized forcooling, while the cooling interface on another side of the planarwaveguide 702 could be optimized for minimizing Amplified SpontaneousEmission (ASE).

In this particular example, one cooling interface is implemented using aliquid cooling manifold 704. The liquid cooling manifold 704 couldsupport any suitable direct liquid cooling technique. In this example, amulti jet liquid impingement cooler is used, although other approaches(such as ultra-thin liquid thermal-optical interface cooling or otherapproaches described above) may be used. A seal 706, such as an O-ringor other type of seal, helps to prevent leakage of the cooling liquidout of the liquid cooling manifold 704. Another cooling interface isimplemented in FIG. 7 using a dry conductive cooler 708 that could bethermally coupled to the planar waveguide 702 using a thermal-opticinterface (TOI) material 710. One example of this approach is describedin U.S. Patent Publication No. 2014/0268309 (which was incorporated byreference earlier).

Note that in this example, the planar waveguide 702 could be replaced byanother planar waveguide, such as the planar waveguide 500 of FIG. 5 orother asymmetrical or symmetrical waveguide. The ability to cooldifferent sides of a planar waveguide, particularly different sides ofan asymmetrical planar waveguide, using different cooling technologieshelps to improve the overall cooling efficiency in a PWG assembly.

As shown in FIG. 8, a PWG assembly 800 includes a planar waveguide 802and endcaps 804. The endcaps 804 could denote passive guiding endcaps orunguiding endcaps depending on the design on the planar waveguide 802.The endcaps 804 could be used at an input end and/or an output end ofthe planar waveguide 802. Also, side claddings 806 may be bonded orotherwise attached to the planar waveguide 802 along the longer sides ofthe planar waveguide 802, allowing the endcaps 804 to be used inconjunction with the side claddings 806 to provide a continuous mountingregion. Any doping of the side claddings 806 produces negligibleabsorption at the pump and signal wavelengths, so little or no lasingoccurs within the side claddings 806. The side claddings 806 could beformed from any suitable material(s) and in any suitable manner. Forexample, the side claddings 806 could be formed from the samematerial(s) as the cladding layer(s) of the planar waveguide 802. Toavoid thermal stress under operation, the side claddings 806 may have asubstantially similar coefficient of thermal expansion to elements ofthe planar waveguide 802, such as the thicker cladding layer of theplanar waveguide 802.

A cooling manifold 808 is used here to cool the planar waveguide 802.The cooling manifold 808 represents any suitable structure fordelivering cooling fluid to a planar waveguide, such as a multi jetliquid impingement cooler or other direct liquid cooler such as thosedescribed above. A seal 810 (such as an O-ring) is positioned betweenthe PWG assembly 800 and the cooling manifold 808 to help preventleakage of the cooling fluid. The endcaps 804 and the side claddings 806can be used here to facilitate a more reliable seal for the coolingmanifold 808. For example, planar waveguides tend to be more delicatestructures, and reducing or eliminating contact between the planarwaveguide 802 itself and the seal 810 can help to protect variousstructures in FIG. 8. Note, however, that the side claddings 806 couldbe used without the endcaps 804. Also note that while not shown, thesame cooling interface or a different cooling interface could be used onthe top side of the planar waveguide 802 in FIG. 8.

Other types of cooling interfaces could also be used with a planarwaveguide having side claddings. For example, as shown in FIG. 9, a PWGassembly 900 includes a planar waveguide 902, endcaps 904, and sidecladdings 906. These components may be the same as or similar tocorresponding components in FIG. 8. In FIG. 9, one cooling interfaceincludes an aperture plate 908, which denotes a plate having an aperture(opening) in the central portion of the plate. The aperture plate 908can be bonded to the side claddings 906 and to the endcaps 904 (ifpresent). The aperture plate 908 provides a robust surface for an O-ringor other seal for a cooling manifold, placing the O-ring farther awayfrom the input signal and high power output beam. The other coolinginterface in FIG. 9 is formed using a solid plate 910, which could bebonded or otherwise attached to the planar waveguide 902 using the side′claddings 906 and the endcaps 904 (if present). Any suitable bondingcould be used here, such as an adhesive, epoxy, glass solder, metalsolder, hydroxyl bond, or covalent bond. The use of a solid plate 910can facilitate cooling and structural stiffness on the side opposite theliquid cooling manifold. Both the aperture plate 908 and the solid plate910 could have a substantially similar coefficient of thermal expansionto element(s) of the planar waveguide 902.

As shown in FIGS. 10 and 11, a PWG assembly 1000 includes a planarwaveguide 1002 bonded or otherwise secured to two endcaps 1004 a-1004 b,which can be secured to a housing 1006. The endcaps 1004 a-1004 binclude slots through which the planar waveguide 1002 can extend,allowing portions of the planar waveguide 1002 to remain partiallyexposed. Any suitable mechanism can be used to secure the planarwaveguide 1002 to the endcaps 1004 a-1004 b, such as metal solder, glasssolder, or ultraviolet (UV) curing adhesive. The endcaps 1004 a-1004 bcreate a seal around the ends of the planar waveguide 1002, but theendcaps 1004 a-1004 b can be somewhat compliant to allow for thermalexpansion. Coolers 1008 a-1008 b are also bonded or otherwise secured tothe housing 1006, and seals can be formed around the sides of thehousing 1006. This creates a sealed internal cavity within which theplanar waveguide 1002 is substantially located and through which coolingfluid can flow.

Each cooler 1008 a-1008 b here includes multiple channels 1010-1012 anda flow controller 1014. For each cooler 1008 a-1008 b, one of thechannels 1010-1012 can allow passage of a cooling fluid into theinterior of the housing 1006, and another of the channels 1010-1012 canallow exit of the cooling fluid from the interior of the housing 1006.The cooling fluid flows between the channels 1010-1012 within theinterior of the housing 1006 through a passage defined by the flowcontroller 1014. The flow controller 1014 here creates a substantiallylinear passageway for the cooling fluid to flow past the planarwaveguide 1002 along the length of the planar waveguide 1002. Note,however, that the flow controller 1014 could have any other suitablesize and shape. The flow controller 1014 includes any suitable structurefor defining a desired passageway for cooling fluid. In this example,the flow controller 1014 has a design in which the flow controllerflares outward.

In the embodiment in FIGS. 10 and 11, the same cooling interface is usedon both sides of the planar waveguide 1002. However, as with previousembodiments, different cooling interfaces could be used on differentsides of the planar waveguide 1002. For example, the cooler 1008 a couldbe used on one side of the planar waveguide 1002, and a slab cooler orother type of cooler could be used on the other side of the planarwaveguide 1002. As with prior embodiments, the materials used in thecooling interfaces could have a substantially similar coefficient ofthermal expansion to elements of the planar waveguide 1002.

As shown in FIG. 12, a PWG assembly 1200 includes a planar waveguide1202, which in this example is implemented using the core region 602 andthe cladding layers 604 a-604 b of FIG. 6 (although other planarwaveguides could be used). The planar waveguide 1202 here is asymmetric,and as a result the thinner cladding layer 604 b may increase intemperature less than the thicker cladding layer 604 a during operationof the planar waveguide 1202. The thinner cladding layer 604 b is cooledusing the dry conductive cooler 708 and the TOI material 710 of FIG. 7(although other cooling techniques could be used with the thinnercladding layer 604 b).

The thicker cladding layer 604 a in this example is uncooled, meaningthere is no active cooling used in conjunction with the thicker claddinglayer 604 a. However, due to the refractive index of air on the thickercladding layer 604 a, a large fluorescence heat load could be created onthe cooler 708. Placing a transparent material 1204 with a proper indexof refraction, such as sapphire or fused silica, on the thicker claddinglayer 604 a allows fluorescence to “leak out” of the PWG assembly 1200,thereby reducing heat loads on the cooler 708 and reducing temperatureand stress in the PWG assembly 1200. Note that any suitable techniquecould be used to secure the transparent material 1204 to the thickercladding layer 604 a, such as clamping.

Because the cladding layer 604 b is thinner than the cladding layer 604a, there will be less of a temperature rise through the thickness of thecladding layer 604 b. Therefore, the surface of the cladding layer 604 bcan be cooled to help maintain the planar waveguide 1202 at a lowertemperature than if the surface of the cladding layer 604 a was cooled.

Although FIGS. 5 through 12 illustrate examples of arrangements forsupporting and/or cooling a planar waveguide of a high-power lasersystem, various changes may be made to FIGS. 5 through 12. For example,each figure may include a symmetric or asymmetric planar waveguide ofany suitable design. Also, a planar waveguide could include any singleinstance of the various cooling interfaces shown and described above orany other suitable cooling interface, or a planar waveguide couldinclude multiple instances of one or more of the cooling interfacesshown and described above or any other suitable cooling interfaces. Ifmultiple cooling interfaces are used, those cooling interfaces may ormay not be identical.

High-power laser systems, such as the ones described above, could beused in a large number of military and commercial applications. Thefollowing discussion provides a description of various examplecommercial applications. However, the following discussion does notlimit this disclosure to any particular applications.

High-power laser systems could find use in commercial miningapplications, such as in drilling, mining, or coring operations. Forinstance, high-power laser systems could be used to soften or weaken anearth bed prior to drilling through the earth bed using drill bits. Thiscould allow for fewer drill bit changes and extended lifetimes andreliabilities of the drill bits. Here, free-space propagation of ahigh-power laser beam from an output window of a laser system could beused, allowing deeper penetration at further distances compared toconventional fiber lasers.

High-power and high-beam quality laser systems could also find use inremote laser welding, cutting, drilling, or heat treating operations,such as in industrial or other automation settings. The use of ahigh-power and high-beam quality laser system allows the processing ofthicker materials to occur at larger working distances from the lasersystem while minimizing the heat-affected zone and maintaining verticalor other cut lines. Among other things, this helps to support welding orcutting operations where proximity to the weld or cut site is difficultor hazardous. It also helps to protect the laser system and possibly anyhuman operators from smoke, debris, or other harmful materials.

High-power laser systems could further find use in construction anddemolition operations. Example operations could include metalresurfacing or deslagging, paint removal, and industrial demolitionoperations. High-power laser systems can be used to ablate material muchfaster and safer compared to conventional operations. As a particularexample of this functionality, high-power laser systems could be used tosupport demolition of nuclear reactors or other hazardous structures.Here, the high-power laser systems could be used to cut throughcontaminated structures like contaminated concrete or nuclearcontainment vessels or reactors from long distances. This helps to avoidthe use of water jet cutting or other techniques that create hazardouswaste, such as contaminated water. It also provides improved safetysince human operators can remain farther away from contaminatedstructures being demolished.

A number of additional applications are possible. For example,high-power laser systems could find use in power beaming applications,where high-power laser beams are targeted to photovoltaic (solar) cellsof remote devices to be recharged. High-power laser systems could finduse in hazardous material (HAZMAT) applications, where the laser systemsare used to heat and decompose hazardous materials into less harmful ornon-harmful materials.

In some embodiments, various functions described in this patent documentare implemented or supported by a computer program that is formed fromcomputer readable program code and that is embodied in a computerreadable medium. The phrase “computer readable program code” includesany type of computer code, including source code, object code, andexecutable code. The phrase “computer readable medium” includes any typeof medium capable of being accessed by a computer, such as read onlymemory (ROM), random access memory (RAM), a hard disk drive, a compactdisc (CD), a digital video disc (DVD), or any other type of memory. A“non-transitory” computer readable medium excludes wired, wireless,optical, or other communication links that transport transitoryelectrical or other signals. A non-transitory computer readable mediumincludes media where data can be permanently stored and media where datacan be stored and later overwritten, such as a rewritable optical discor an erasable memory device.

It may be advantageous to set forth definitions of certain words andphrases used throughout this patent document. The terms “application”and “program” refer to one or more computer programs, softwarecomponents, sets of instructions, procedures, functions, objects,classes, instances, related data, or a portion thereof adapted forimplementation in a suitable computer code (including source code,object code, or executable code). The term “communicate,” as well asderivatives thereof, encompasses both direct and indirect communication.The terms “include” and “comprise,” as well as derivatives thereof, meaninclusion without limitation. The term “or” is inclusive, meaningand/or. The phrase “associated with,” as well as derivatives thereof,may mean to include, be included within, interconnect with, contain, becontained within, connect to or with, couple to or with, be communicablewith, cooperate with, interleave, juxtapose, be proximate to, be boundto or with, have, have a property of, have a relationship to or with, orthe like. The phrase “at least one of,” when used with a list of items,means that different combinations of one or more of the listed items maybe used, and only one item in the list may be needed. For example, “atleast one of: A, B, and C” includes any of the following combinations:A, B, C, A and B, A and C, B and C, and A and B and C.

The description in this patent document should not be read as implyingthat any particular element, step, or function is an essential orcritical element that must be included in the claim scope. Also, none ofthe claims is intended to invoke 35 U.S.C. § 112(f) with respect to anyof the appended claims or claim elements unless the exact words “meansfor” or “step for” are explicitly used in the particular claim, followedby a participle phrase identifying a function. Use of terms such as (butnot limited to) “mechanism,” “module,” “device,” “unit,” “component,”“element,” “member,” “apparatus,” “machine,” “system,” “processor,”“processing device,” or “controller” within a claim is understood andintended to refer to structures known to those skilled in the relevantart, as further modified or enhanced by the features of the claimsthemselves, and is not intended to invoke 35 U.S.C. § 112(f).

While this disclosure has described certain embodiments and generallyassociated methods, alterations and permutations of these embodimentsand methods will be apparent to those skilled in the art. Accordingly,the above description of example embodiments does not define orconstrain this disclosure. Other changes, substitutions, and alterationsare also possible without departing from the spirit and scope of thisdisclosure, as defined by the following claims.

What is claimed is:
 1. An apparatus comprising: a planar waveguideconfigured to receive and amplify optical signals, the planar waveguidecomprising: a core region; first and second coating or cladding layersdisposed on opposite sides of the core region; and one or more endcapsdisposed between the first and second coating or cladding layers at oneor more ends of the core region; wherein the core region comprises atleast one material and is doped with at least one active ion species,and wherein each endcap comprises the at least one material and is notdoped with any active ion species that creates substantial absorption atpump and signal wavelengths; and wherein each endcap is doped with atleast one dopant to change an index of refraction of the at least onematerial forming the endcap.
 2. The apparatus of claim 1, wherein theone or more endcaps comprise first and second endcaps, the first endcappositioned at an input end of the planar waveguide, the second endcappositioned at an output end of the planar waveguide.
 3. The apparatus ofclaim 1, wherein: the core region has a rectangular cross-section thatis narrower in a fast-axis dimension and wider in a slow-axis dimension;and the planar waveguide is configured to guide the optical signals inthe fast-axis dimension.
 4. The apparatus of claim 1, wherein eachendcap comprises a passive guiding endcap.
 5. The apparatus of claim 1,wherein the first and second coating, or cladding layers are asymmetric.6. An apparatus comprising: a planar waveguide configured to receive andamplify optical signals, the planar waveguide comprising a core regionand at least one coating or cladding layer disposed on the core region;first and second side claddings disposed on opposite sides of the planarwaveguide, the opposite sides representing longer sides of the planarwaveguide; first and second endcaps, the first endcap positioned at aninput end of the planar waveguide, the second endcap positioned at anoutput end of the planar waveguide; and at least one of a cooler and aseal that contacts the first and second side claddings and the first andsecond endcaps.
 7. The apparatus of claim 6, wherein: the core regionhas a rectangular cross-section that is narrower in a fast-axisdimension and wider in a slow-axis dimension; and the planar waveguideis configured to guide the optical signals in the fast-axis dimension.8. The apparatus of claim 6, wherein each endcap comprises a passiveguiding endcap.
 9. The apparatus of claim 6, wherein each endcap isdoped with at least one dopant to change an index of refraction of atleast one material forming the endcap.
 10. The apparatus of claim 6,wherein the at least one coating or cladding layer comprises first andsecond coating or cladding layers.
 11. The apparatus of claim 10,wherein the first and second coating or cladding layers are asymmetric.12. The apparatus of claim 6, wherein multiple coolers contact the firstand second side claddings and the first and second endcaps.
 13. Anapparatus comprising: a planar waveguide configured to receive andamplify optical signals, the planar waveguide comprising a core regionand at least one coating or cladding layer disposed on the core region;first and second endcaps sealed to the planar waveguide and to ahousing; and one or more coolers sealed to the housing and configured tocool the planar waveguide; wherein at least one of the one or morecoolers is configured to receive a coolant and to direct the coolantthrough a passageway defined within an interior space of the housing,the passageway comprising a substantially linear passageway along alength of the planar waveguide.
 14. The apparatus of claim 13, whereinthe at least one of the one or more coolers is configured to receive thecoolant through a first channel, direct the coolant through thepassageway defined by a flow controller, and provide the coolant througha second channel.
 15. The apparatus of claim 13, wherein each of thefirst and second endcaps includes a slot through which the planarwaveguide extends such that end portions of the planar waveguide remainexposed.
 16. The apparatus of claim 14, wherein: the one or more coolerscomprise multiple coolers; and each cooler comprises the first channel,the flow controller, and the second channel.
 17. An apparatuscomprising: a planar waveguide configured to receive and amplify opticalsignals, the planar waveguide comprising: a core region having two majorsurfaces on opposite sides of the core region, two minor surfaces thatextend between the major surfaces and that are smaller than the majorsurfaces, and two end faces that are smaller than the major and minorsurfaces; first and second coating or cladding layers disposed on thetwo major surfaces on the opposite sides of the core region; and one ormore endcaps disposed between the first and second coating or claddinglayers at one or more ends of the core region; wherein the core regioncomprises at least one material and is doped with at least one activeion species, and wherein each endcap comprises the at least one materialand is not doped with any active ion species that creates substantialabsorption at pump and signal wavelengths.
 18. The apparatus of claim17, wherein the one or more endcaps comprise first and second endcaps,the first endcap positioned at an input end of the planar waveguide, thesecond endcap positioned at an output end of the planar waveguide. 19.An apparatus comprising: a planar waveguide configured to receive andamplify optical signals, the planar waveguide comprising: a core regionhaving two broader surfaces on opposite sides of the core region; firstand second coating or cladding layers disposed on the two broadersurfaces on the opposite sides of the core region; and one or moreendcaps disposed between the first and second coating or cladding layersat one or more ends of the core region; wherein the core regioncomprises at least one material and is doped with at least one activeion species, and wherein each endcap comprises the at least one materialand is not doped with any active ion species that creates substantialabsorption at pump and signal wavelengths; and wherein each endcap isdoped with at least one dopant to change an index of refraction of theat least one material forming the endcap.
 20. The apparatus of claim 17,wherein each endcap comprises a passive guiding endcap.
 21. Theapparatus of claim 17, wherein the first and second coating or claddinglayers are asymmetric.
 22. The apparatus of claim 17, wherein; the coreregion has a rectangular cross-section that is narrower in a fast-axisdimension and wider in a slow-axis dimension; and the planar waveguideis configured to guide the optical signals in the fast-axis dimension.23. An apparatus comprising: a planar waveguide configured to receiveand amplify optical signals, the planar waveguide comprising: a coreregion; first and second coating or cladding layers disposed on oppositesides of the core region; and one or more endcaps disposed between thefirst and second coating or cladding layers at one or more ends of thecore region; a first cooler disposed on or adjacent to the first coatingor cladding layer and configured to cool the planar waveguide; and asecond cooler disposed on or adjacent to the second coating or claddinglayer and configured to cool the planar waveguide.
 24. The apparatus ofclaim 23, wherein the first and second coating or cladding layers areasymmetric.
 25. The apparatus of claim 24, wherein: the first coating orcladding layer is thinner than the second coating or cladding layer; thefirst cooler comprises a direct liquid cooler; and the second coolercomprises a conductive cooler.